Ground fault circuit interrupting (GFCI) devices traditionally utilize a differential current transformer to sense current imbalances in the line and neutral conductors of a power distribution circuit, as occasioned by ground leakage current from the line conductor returning to the source through an unintended ground circuit path other than the neutral conductor. To prevent injurious electrical shock, the differential current transformer must develop a signal voltage of sufficient magnitude to enable a signal processor to initiate circuit interruption when the current differential in the line and neutral conductors is as low as 5 milliamps. A problem arises in sensing the true magnitude of this ground leakage current if the neutral conductor is subjected to a low impedance ground fault adjacent the load. Since the neutral conductor is also grounded at the source in conventional wiring installations, such double grounding of the neutral conductor could create a situation where a portion of this ground leakage current returns to the source through the neutral conductors. As a consequence, the current differential showing up in the differential current transformer would not be truly indicative of the magnitude of the ground leakage current. Thus, a sufficiently low impedance neutral to ground fault has the potential of desensitizing the differential current sensor such that the GFCI device would trip only in response to considerably higher ground leakage current levels. It is for this reason that Underwriters Laboratories requires that GFCI devices have the capability of interrupting the circuit in the event of a desensitizing ground fault on the neutral conductor.
The conventional approach toward coping with such desensitizing neutral-ground faults is to utilize a second, so-called neutral transformer having a secondary winding connected in series with the neutral side of the distribution circuit. If the neutral conductor experiences a low impedance ground fault, it becomes a closed loop secondary winding, and driving of the neutral transformer primary will produce a current flow in this secondary loop. If the neutral ground fault impedance is sufficiently low as to have a significant desensitizing effect on the response of the GFCI to ground leakage current, the current induced in this secondary loop is of sufficient magnitude to create the requisite current imbalance in the differential transformer for initiating a trip function.
The addition of this second transformer adds significantly to the complexity and cost of GFCI design, and also takes up valuable space which is at a premium when faced with the task of packaging a GFCI module in circuit breakers and outlet receptacles capable of being installed in existing enclosures.
It is accordingly an object of the present invention to provide an improved ground fault circuit interrupting device capable of responding to both high impedance ground faults on the line conductor and low impedance ground faults on the neutral conductor of a conventional electrical power distribution circuit.
An additional object is to provide a ground fault circuit interrupting device of the above character which is capable of responding to desensitizing ground faults on the neutral conductor without requiring a neutral transformer.
An additional object of the present invention is to provide a ground fault circuit interrupting device of the above character which is compact, simple in design, and inexpensive to manufacture.
Other objects of the invention will in part be obvious and in part appear hereinafter.